The potential for Mars' water to be lost into space is greater during the planet's warm and stormy seasons, according to a new study. This seasonal variation likely regulated the rate at which the red planet lost its once-abundant quantities of liquid water, the authors suggest. Ancient alluvial fans, dry lake beds and winding river valleys clearly demonstrate that the surface of Mars was once a far wetter place. The vast majority of the liquid water that once flowed freely across the Martian surface was later lost to space, leaving behind a dusty, desert-like landscape. Still, water persists on Mars. Today, almost all of the water that remains on Mars is frozen and locked up in the planet's polar ice caps; however, small amounts of water vapor and clouds of water ice crystals are still present in the atmosphere. Although atmospheric water vapor represents a minute fraction of Mars' remaining water, it can potentially dissipate into space if it rises high enough into the upper atmosphere, which controls the rate of the planet's continued desiccation. Anna Fedorova, Franck Montmessin, Oleg Korabley and colleagues used measurements from the ExoMars Trace Gas Orbiter spacecraft to investigate the vertical distribution of water throughout the Martian atmosphere. They found that while the planet's massive dust storms impacted water vapor distributions, seasonal changes were the most dominant regulator of atmospheric water. During the warmest parts of Mars' orbit, large portions of the Martian atmosphere became supersaturated, allowing water to continue to reach the upper atmosphere. The authors conclude that the higher-than-expected amount of water in the upper atmosphere during this season likely dominated the loss of water into interplanetary space.